1. Field of the Invention
The present invention relates to vacuum processing equipment. More particularly, the present invention relates to the creation and maintenance of high vacuum environments in which substrates, including semiconductor substrates, may be degassed or cleaned before processing by sputtering or other film layer deposition or etching processes. More particularly still, the present invention relates to a method of removing contaminants outgassed from a substrate in a degas chamber through placement of a getter pump in the de-gas chamber.
2. Background of the Invention
Thin film deposition apparatus and techniques are used, among other techniques, to provide film layers on semiconductor substrates. An environment in which desired materials are transported in an ultrahigh vacuum to condense on a clean substrate is a preferred processing technique in the fabrication of microdevices. One well known prior art deposition process is sputtering where substrates are typically moved through load locks and into high vacuum processing chambers which enclose the substrate and a target composed of the material desired to be deposited on the substrate. A negative voltage applied to the target excites into a plasma state an inert gas (typically argon) supplied to the chamber. Ions from the plasma bombard the target and eject particles of target material from the target toward the substrate. These particles deposit on the substrate to form the desired film. In some sputtering process applications, the substrate may be heated to temperatures on the order of about 350xc2x0 C. to about 510xc2x0 C. or higher, to reflow a film layer deposited on the substrate by sputtering.
The low pressure, high temperature environments typically utilized in deposition processes cause outgassing of contaminants in the substrates, as well as the components of the deposition chamber. These contaminants, such as hydrogen (H2), water (H2O) and air (mostly O2 and N2), are detrimental to the film layer which is deposited onto the substrate. Therefore, processes for removing contaminants prior to processing have been developed to reduce the problem of contamination in processing environments. Such processes typically are confined to degas chambers located near the front of a cluster tool or otherwise in the preliminary steps of substrate processing.
Typically, substrates are housed in degas chambers and subjected to processing conditions, such as low pressure and high temperature, for a period of time prior to processing of outgas contaminants therefrom. Such processes typically include heating the substrates to a temperature at least as high as the processing temperature to which the wafer will be subjected in a low pressure chamber environment. The result of heating the substrate at low pressure is outgassing of contaminants such as hydrogen, H2O and air from the substrate. The theory being that by subjecting the substrate to at least the processing temperature at low pressure all contaminants that would be outgassed during processing at this temperature and pressure will have already been removed from the substrate by the time processing occurs. Following outgassing of contaminants, the substrate is moved through a load lock and into a processing chamber where processing occurs.
Having the substrate surface atomically clean before processing can be of vital importance for adhesion of subsequently deposited films and determining other physical properties that are dependent on the interface between the substrate and the film, such as electrical contact. However, the contaminants which are outgassed from the substrate must be removed from the degas chamber to maintain the required vacuum level therein, and also to prevent migration out of the degas chamber and into the load lock area between the pre-processing apparatus and the processing apparatus. This problem exists in applications where wafer exchange in and out of the degas chamber is controlled via a slit valve in the degas chamber and where a slit valve is absent.
It is necessary, therefore, that the substrates on which the processing steps are typically performed must first be degassed because the low pressure, high temperature environments in which most processing steps are performed will otherwise outgas contaminants and compromise the integrity of the devices formed on the substrate. Therefore, the substrates must be degassed before introduction into the processing chamber to prevent contamination of both a film subsequently deposited on the substrate and the surface of the substrate of the interface between the film and the substrate.
Processing systems for workpieces such as semiconductor substrates incorporate multiple, isolated vacuum stages between the cassette load lock station and the main vacuum processing chambers. A vacuum gradient is applied between the cassette load lock and the main processing chambers to facilitate the use of a very high degree of vacuum in the processing chambers without lengthy pump down times. Pre-treatment chambers, such as degas chambers, are typically incorporated in the transport paths. One such processing system is described in U.S. Pat. No. 5,186,718, Tepman et al., issued on Feb. 16, 1993, the subject of which is hereby incorporated herein by reference. Pre-treatment chambers can be dedicated to pre-treating (e.g., plasma etch cleaning, vacuum cleaning and/or heating) of the substrates before processing.
A combination of a roughing pump and a cryogenic pump is typically used to provide the vacuum in the degas chamber. Where the chamber is maintained at a temperature of below approximately 300xc2x0 C., the roughing pump can reduce the pressure within the chamber to about 10xe2x88x923 torr, and the cryogenic pump can then reduce the chamber pressure to a pressure on the order of 10xe2x88x928 torr.
To minimize the inclusion of impurities or contaminants in the film deposited on the substrate, it is generally considered desirable to maintain the sputtering chamber at the highest possible vacuum, that is, the lowest possible pressure. However, the greater the vacuum in the chamber and the higher the temperature in the chamber, the greater the influx of undesirable contaminants, such as H2O and air, into the chamber due to outgassing from the substrates.
Because of the low condensation temperature of hydrogen, cryogenic pumps are relatively ineffective at removing hydrogen. The difficulty of maintaining a sufficiently low partial pressure of hydrogen has limited the ability of existing sputtering chambers to operate at high temperatures and ultra-high vacuums.
Sputter deposition of aluminum films on semiconductor substrates is an application in which ultra-high vacuum at high temperatures is especially desirable. The ultra high vacuum is used to achieve outgassing of contaminants and removal thereof from the chamber. The chamber pressure for sputtering, commonly as high as the milli-torr range, is commonly achieved by first evacuating the chamber to an ultra high vacuum pressure as low as the 10xe2x88x928 torr range to remove contaminants from the chamber, and then re-filling (xe2x80x9cback fillingxe2x80x9d) the chamber with a clean process gas at a total chamber pressure regulated at about 10xe2x88x923 torr.
High substrate temperatures are used to enhance the step coverage of the deposited film, that is, to improve the ability of the film to completely fill holes and trenches in the surface of the semiconductor substrate. Higher temperatures promote the diffusion of deposited atoms, called xe2x80x9creflow,xe2x80x9d to fill any gaps in, and equalize the thickness of the film as it is being deposited onto the substrate. Lower chamber pressures promote reflow of the deposition material on the substrate by reducing the occurrence of reflow inhibiting contaminants on the substrate surface. Reflow may occur simultaneously with the deposition of the film layer, at the end of the deposition step, or in a separate chamber dedicated to reflow.
For aluminum reflow, the substrate generally is heated by heating the pedestal which holds the substrate during processing. Consequently, existing sputter deposition chambers have achieved a chamber pressure of about 1.5xc3x9710xe2x88x928 torr at a substrate temperature of 510xc2x0 C., where the pedestal is maintained at a temperature of approximately 580xc2x0 C. At this temperature and pressure, contaminants not previously removed from the substrate during the degas process will be outgassed during processing, thereby compromising the integrity of the devices formed on the substrate.
Therefore, there is a need to provide a method and apparatus for outgassing contaminants from the substrate and removing the contaminants which are outgassed from the substrte to prevent migration of the contaminants out of the degas chamber during wafer exchange or due to the lack of a slit valve. By outgassing and removing the contaminants from the degas chamber, the quality of the deposition layers, and devices formed as a result of such deposition, will be greatly improved.
The present invention is directed to a degas chamber wherein substrates are outgassed and getter material is disposed adjacent to the substrate to remove contaminants, such as hydrogen and H2O which are outgassed from the substrate into the chamber volume and which when outgassed in the processing chamber, might compromise the integrity of devices formed on the substrate, and limit the ability to reach base pressure.
The invention provides a vacuum producing apparatus, an enclosure, a vacuum pump ported to the enclosure and capable of evacuating the enclosure to a first vacuum pressure, and getter material received within the enclosure for chemisorbing reactive gas species in the chamber.
One embodiment of the invention provides an apparatus for maintaining a vacuum pressure in a processing chamber for processing semiconductor substrates having a film layer thereon, comprising: an evacuable enclosure; a heating member to heat a substrate, when received in the enclosure, to a temperature high enough to effect outgassing of contaminants in the substrate; a vacuum pump disposed in communication with the enclosure and capable of reducing the pressure in the enclosure to a first pressure at a substrate outgassing temperature; and getter material disposed in the enclosure and capable of further reducing the pressure within the enclosure from the first pressure to a second pressure below the first pressure at a substrate outgassing temperature.
Another embodiment of the invention provides an apparatus having an evacuable enclosure for receiving a substrate and outgassing contaminants in the substrate, comprising: a vacuum pump ported to the enclosure and capable of evacuating the enclosure to a first vacuum pressure; getter material in communication with the enclosure and capable of reducing the pressure in the enclosure from the first pressure to a second, lower, pressure; and a heater capable of heating the substrate, when received in the enclosure, to a temperature sufficient to outgas contaminants in the substrate.
Yet another embodiment of the invention provides a method of providing a vacuum in a degas chamber, comprising the steps of: evacuating the chamber with a vacuum pump wherein the vacuum pump is alone capable of reducing the chamber pressure to a first chamber pressure; and removing gaseous material from the chamber volume with a getter material so as to reduce the chamber pressure to a pressure less than the first chamber pressure.
Yet another embodiment of the invention provides a method of outgassing a semiconductor substrate at high vacuum and high temperature, comprising: providing a chamber; evacuating the chamber with a vacuum pump wherein the vacuum pump is alone capable of reducing the chamber pressure to a first chamber pressure; absorbing gases in the chamber with a getter material so as to reduce the chamber pressure to a pressure less than the first chamber pressure; providing a semiconductor substrate in the chamber; and raising the temperature of the substrate from an ambient temperature to a temperature high enough to outgas contaminants in the substrate.